Function of a multimodal signal: a multiple-hypotheses test using an electromechanical robot

1. Many animal species communicate using multimodal signals, which are composed of two or more components emitted and interpreted through different sensory modalities. The main types of selective pressures leading to the evolution of multimodal signals are: (1) content-based, when combined components convey information about the signaller, (2) efficacy-based, when combined components increase the efficacy of signal transmission or processing, and (3) inter-signal interaction, when combined components act in concert to elicit a receiver’s response that differs from the response to either components alone.2. We experimentally tested predictions of five competing hypotheses about the selective pressures leading to the evolution of an audiovisual display (aggressive calls + toe flags) emitted during agonistic male-male interactions of the Neotropical frog Crossodactylus schmidti. 3. To simulate these agonistic interactions in the field, we built an electromechanical robot that emitted acoustic and visual stimuli, either combined or in isolation. We assessed male receiver’s responses to the stimuli types in terms of occurrence and frequency of signals emission.4. The hypothesis that received most support was the context hypothesis, which states that one signal provides a context in which a receiver can interpret and respond to a second signal. The main findings supporting this hypothesis are: (1) the receiver’s response to the acoustic and visual stimuli were non-redundant; (2) the multimodal stimulus magnified the receiver’s response; and (3) the frequency of aggressive notes and toe flags covaried in the receiver’s response to the multimodal stimulus.5. We suggest that the addition of toe flags to aggressive calls modulates male-male interactions by providing to the receiver a new context for the interpretation and response to the acoustic signal. This new context may be motivation to fight for territory possession, level of aggressiveness, or intent of territory defence or invasion. Our study provides one of the few evidences of a context function of a multimodal signal with a modulation effect in animal contests. Moreover, our results draw attention to the role of inter-signal selection on the evolution of multimodal signals in species living in heterogeneous habitats, where efficacy-based selection is often considered as a major driver.